Underwater activities, like scuba diving, are gaining more and more popularity. In scuba diving equipment play an important role in enabling the underwater adventures. The diving equipment may be categorized to several bigger groups: breathing related equipment, mobility related equipment, underwater vision equipment and monitoring and navigation related equipment. The breathing equipment comprises e.g. breathing gas tank and one or more regulators providing air, or some other gas, to the diver. Mobility related equipment comprise e.g. suit, fins and buoyancy control devices, such as buoyancy compensator. Underwater vision equipment, in turn, comprise such as diving mask and dive lights. Monitoring and navigation related equipment comprise devices such as dive computers which provide information to the diver as relates to dive time, depth and the subsequent decompression status based on the amount of dissolved inert gasses in the diver's body, amount of gas(es) in the breathing gas tank(s), and positional or heading information, for instance. Thus, the monitoring and navigation related equipment may for example comprise one or more gauges and compass coupled to the other equipment in a known manner in order to provide the information. The previous list of scuba divers' equipment is not comprehensive and it may depend on the environment and purpose of the diving, e.g. recreational or professional.
The underwater navigation is one of the most challenging tasks to do during the dive regardless of diver's skills or experience. There are several reasons causing challenges to the navigation task, such as human sensory system cannot completely adapt to underwater environment, visibility under the water is more or less limited, there are many tasks concurrently going on during the dive and navigation does not get the attention required, limitations of the equipment and so on.
Traditionally, the underwater navigation is based on a use of compass and the diver mentally creates a spatial map based on the compass information.
Additionally, the divers may derive some distance information by counting kick-cycle every time the diver kicks with one particular leg and evaluating the propagation distance per kick, or kick cycle. Thus, the total distance is evaluated with a great uncertainty. A rough level of navigation may be achieved by combining the compass information with the manually calculated distance information. The manual navigation based on a use of compass and counting kick-cycles demands a great deal of uninterrupted attention in underwater environment and is often hampered when the diver attention is needed elsewhere.
Some other approaches for underwater navigation are also taken. First one is a utilization of ultrasound based systems. Typical ultrasound based underwater navigation device consists of a diver unit worn on the user's wrist, or carried as a separate hand held device, and a transceiver unit. The transceiver unit is often embodied as a statutory “homing” instrument that is placed in the water at the entry point. These systems use algorithms that control relaying of coded or non-coded ultrasonic signals between the two devices while simultaneously performing all direction and distance calculations. Second type of underwater navigation solutions are based on a cable communication between a surface float and a diver. The location information obtained by a satellite navigation GPS receiver equipped surface float is dispatched to the diver by the means of an underwater cable, thus enabling underwater navigation to some extent.
All these solutions suffer from technical or safety shortcomings. First, their dependence on a buoy support curbs their operation range and the usage of underwater cable entails a risk of entanglement which is a safety concern itself. Second, a need for the line-of-sight in use of ultrasound systems limits them to open water diving applications and achieving a high overall reliability is compromised for a number of reasons. Sea water is not uniform in pressure, temperature or salinity which all have an important effect on underwater sound propagation at angles relative to the buoy. A thermocline layer in the sea water can act as a horizontal waveguide for sound wave and can create a total blockage for the sound signal between the surface and the bottom. Combined with problems of triangulating from a small floating raft, the task of reaching mandatory reliability and accuracy becomes even more challenging. At their best, ultrasound navigation systems are only able to provide the range and bearing to the homing device which limits their potential as true navigation devices. Finally, ultrasound based devices tend to be bulky, power hungry and generate ultrasound noise potentially harmful to marine life.
Thus, there is need to develop solution for underwater navigation which at least partly mitigate the drawbacks of the known solutions.